Systems and methods for non-contact measuring sputtering target thickness ultrasonics

ABSTRACT

A method and apparatus for ultrasonically measuring the thickness of sputter targets of varying shapes. An immersion bubble ( 32 ) and transducer ( 36 ) provide pulses to a front surface ( 24 ) and a front surface/bonded surface ( 26 ) interface of a target. The pulses generate reflected echoes that are converted to electric signals. By measuring the difference in time that the electric signals occur the thickness of the target may be approximated to identify whether the thickness of the target is appropriate for use. The system includes a sputter track ( 15 ), specimen ( 20 ), chuck ( 28 ), nozzle ( 34 ), columns ( 60 ), opening ( 62 ), inlet ( 70 ), cable ( 58 ), gauge ( 59 ), turret ( 90 ), position ( 92 ), remote PC controller ( 110 ), electrical line ( 112 ), and rear part ( 84 ).

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/465,190 filed Apr. 24, 2003.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to non-contact ultrasonic thickness measurementof sputtering targets bonded to a backing plate using immersion bubblertechnique and data acquisition over-sampling.

2. Description of Related Art

In the fabrication of integrated circuits and other electronic,opto-electronic, microwave, and MEM devices, multiple deposition andetch processes are performed in sequence to fabricate the desiredelectronic structures or devices. The current trend in fabrication hasbeen to improve the performance and reliability of devices withsimultaneous reduction in manufacturing cost. The ultimate goal is tofabricate devices in a way that combines improved performance (speed andcapacity), with improved cost efficiency of manufacturing process.Manufacturing cost can be kept under control in a number of ways,particularly by reducing the cost of consumables used in the process.One of such consumables is a sputtering target. The cost of thesputtering target can be reduced substantially by replacing the part ofexpensive target material which is not a part of the sputter erosionprocess, with less expensive commercially available “backing” material.The “backing” material, in addition to cost reduction, provides improvedmechanical, thermal, and even electrical properties of the target. Thisbecomes of particular importance for targets made of mechanically softmaterials. These targets can be deformed by thermo-mechanical stressesapplied to the target during sputter-related heat load cycling. Incontrast, a backing plate made of “backing” material provides extramechanical stiffness and improved thermal conductance.

Backing material can be attached to the target in a number of ways.However, only three techniques, namely, mechanical, diffusion, or solderbonding, are of practical interest for target-to-backing plate joining.All three techniques require high or elevated pressure and high orelevated temperature to complete the bonding process. The drawback ofthese techniques is the significant difficulty in maintaining thepre-designed shape of the bond interface, for example, the flatness forplanar targets. In many cases, when different target and “backing”materials are used, mismatches in thermal expansion coefficients of thedifferent materials causes the bond interface to deflect from anoriginally predefined shape. The mechanical flattening which usuallyfollows the bonding process is thus not always capable of flattening thebond interface to a satisfactory level. Therefore, in many cases only apartial correction of deflection of bonded interface is achieved thatresults in target thickness variations all over the target after sputtersurface machining. The thickness variations, in turn, require closemonitoring and measuring. Failure to determine the target minimumthickness may result in catastrophic performance of the target when thetarget sputters through the bond interface into the backing plate,causing contamination in sputtered films.

Attempts to use designing means to change the shape of pre-bondedsurfaces to compensate for bonding-related deflection has shown mixedresults. On the other hand, modeling, for example, by using finiteelement analyses, does not provide a satisfactory prediction for bondinterface deflection due to many uncontrolled variables, which aretypically not accounted for during analysis.

Therefore, there remains a need to measure the actual thickness of thetarget between front surface and the bonded interface. The conventionaltechnique for thickness measurements of bonded assemblies is theultrasonic NDT. A number of portable and stationary thicknessmeasurement instruments or gauges are available from many NDT equipmentmanufacturers. The typical ultrasonic thickness gauge comprises anultrasonic piezoelectric transducer electrically connected to anelectronic block comprising, in turn, a pulser, a receiver, and a signalprocessor, which are controlled by the gauge's internal microcontroller.

The transducer, when excited by a short electric pulse from the pulser,generates a burst of high frequency mechanical vibrations or soundwaves. This sound burst or pulse propagates through the specimen if thespecimen is ultrasonically coupled to the transducer. The sound pulse,when it reaches the bond interface, bounces back to the transducer inthe form of an echo. The transducer converts the echo back into anelectric signal. The electric signal is processed by the gauge, whichcalculates the thickness of the specimen. When the thickness iscalculated it is displayed and transferred to the remote controller ifthe gauge is equipped with a serial, USB, or other type of port.

Typical ultrasonic thickness gauges operate in the “Pulse/Echo” mode, bytiming precisely the reflection of the echo bounced back at normalincidence from the reflecting surface such as the bond interface. If thegauge is calibrated to the speed of sound in the test material then thethickness is determined by an internal calculation performed by thegauge processor using the following relationship [Ref.1]:  Thickness=V(t−t ₀)/2

where:

-   -   V—the velocity of sound in the material,    -   t—the measured transit time of sound pulse,    -   t₀—the zero offset factor (to correct for transducer internal        delay, cable delay, and other fixed delays).

A typical gauge can measure thickness in three modes. Mode 1 is usedwith contact transducers when the transducer is directly coupled to thesurface of the specimen. In this mode, the transit time is measuredbetween a main bang MB pulse and a first returning echo. This method issimplest and it is frequently used for manual thickness measurementswhen the specimen is relatively thick, and only a few thickness datapoints are required to collect. Modes 2 and 3 are used with delay line,or immersion, transducers for the specimens of any, but preferably smallor moderate, thickness when improved measurement accuracy is required.In Mode 2, the transit time is measured between the front surface andthe first backwall (or bond interface) echoes, while in Mode 3 thetransit time is usually measured between two consecutive echoesfollowing the front surface echo. It is important to know that Mode 2 ispreferred for materials with a higher sound attenuation, such as copper,cobalt, tantalum, or WTi while Mode 3 (which is most accurate among allthree modes) is preferred for low attenuated materials such as aluminum,titanium, or tungsten.

Implementation of Modes 1, 2, or 3, when the transducer (with or withoutdelay line) is directly coupled to a target surface, is limited toscratch resistant materials, since only a droplet of water can be usedfor target coupling. As seen frequently in practice, a thin layer ofwater does not provide an adequate protection for target surface,particularly of soft materials such as aluminum or copper, fromscratching. Another drawback of direct contact coupling is that manualoperation depends on operator hands-on experience. A still furtherdrawback of direct contact coupling is the occasional difficulty infinding a region of the target with a minimal thickness. However, thedirect contact coupling has one important advantage, namely, compactnessand mobility, that makes it a preferred technique for use in-situ whenthe part remains attached to the chuck of the machining tool. Thissimplifies testing and reduces the overall test time.

Non-contact immersion Modes 2 and 3 are designed to overcome limitationsof contact methods providing non-scratching, accurate, and automatedmethods of testing. Immersion thickness testing can be done in two ways.It can be done by submerging the entire target assembly and thetransducer into a tank with de-ionized (DI) water where a stationarycolumn of coupling water between target and transducer is formed. Theadvantage of this technique is the ability of using conventional C-Scantechnology and equipment. The disadvantage of this technique is therelatively high cost since several steps are required to complete thetest. Steps include removing the target from machining tool and placingit into a C-Scan tank for testing, then replacing the target back to themachining tool to complete machining. After-test machining is alsorequired, at least as a refinishing measure, to remove thehydro-oxidation caused by extended exposure of the target surface to thewater. Aluminum and copper-made targets are among of most susceptible tohydro-oxidation.

The other way of using immersion testing is a bubbler technique. Thebubbler technique may provide a definite advantage for target thicknessmeasurement compared to all previously discussed methods. The soundbeam, in this case, propagates through a column of flowing water, whichimpinges into the target surface. As a result, the water exposure andsubsequently hydro-oxidation can be minimized drastically by reducingthe size of the water contact area and exposure time. This can beachieved by decreasing the diameter of the water contact area and bybringing this area into a continuous moving contact all over the targetsurface. However, there is a limitation frequently imposed byconventional bubbler techniques. The limitation is a lack of spatialresolution. Conventional conveyor-based bubbler techniques, for example,used in metal rolling mills and etc., acquire thickness data at certainspaced intervals usually pre-defined by conveyor speed and dataacquisition rate. For high conveyor speed applications a plurality ofpositions from where the thickness data are sampled, can be separated belengthy intervals that pose a danger of missing the positions with acritical minimum thickness. This is absolutely not acceptable forsputtering target applications. The region of a target with a minimumthickness should always be detected since the minimum thickness is amongthe most critically controlled target geometrical parameters, whichgoverns pass/fail criterion of the target. Conventional bubblertechniques have another drawback, which may interfere with test remoteoperation. This additional drawback is the possibility of interruptionin the data acquisition process due to occasional discontinuity in thewater flow, especially for small bubbler apertures when a chain of airbubbles is formed in the water supply stream.

Therefore, there is still a need in the art for precision, low cost,non-contact, automated, ultrasonic target thickness measurementtechnique performed in-situ inside a machining tool.

SUMMARY OF THE INVENTION

This need and others are addressed by the bubbler-based, automated,non-contact ultrasonic thickness measurement method comprising the stepsof sequentially irradiating rotating sputtering targets with sonicenergy at normal incidence at a plurality of positions on the surface ofthe target; detecting echoes induced by the sonic energy and reflectedfrom both surfaces of the target with a data acquisition frequency notsynchronized with rotational speed of the target; measuring soundtransit time between two consecutive echoes and calculating thickness bymultiplying known sound velocity of target material by one half ofmeasured transit time of sound pulse, corrected for zero offset factor;sending every value of the thickness data as soon as it is calculated toa remote controller; merging all values of thickness data points to formone sequential file of a statistically representative group of thicknessvalues; analyzing this file of statistically representative group ofthickness values to extract the value of target minimum thickness.

Unlike the prior art, the method of the present invention provides atarget thickness measurement technique, which is always able to find andmeasure target minimum thickness. This is achieved by over-sampling ofthe target thickness data by multiple and repeated sampling from thesame region of the target confined by a plurality of positions along thesame circumferential path of constant radius formed by a number ofconsecutive revolutions. The over-sampling is achieved by irradiatingthe same region of the rotating target during more than one revolutionusing a stationary bubbler and a data acquisition frequency, which isnot synchronized with the target RPM. In this case for every consecutiverevolution a plurality of positions from where the data are collecteddoes not repeat itself. Merging of all the thickness data points fromeach of the plurality of positions into one sequential file provides astatistically representative number of almost all-possible positions fora specific circumferential path. This improves the probability offinding and measuring the target minimum thickness.

It is another object of the invention to provide the apparatus forautomated non-contact in-situ target thickness measurements. Theapparatus consists of an enclosed immersion bubbler with outside shapeand dimensions matching the shape and dimensions of the cutting toolholder of the machining tool; a bubbler body with a water extractingnozzle; a transducer, mounted inside the bubbler axially symmetricalwith a nozzle at a distance from its opening; an electric cableconnecting the transducer with a thickness gauge which, in turn, isremotely connected to a controller; a water supply line connecting thebubbler with a DI water source using at least two valves, connected inseries, one for on/off operation and the second for precision water flowtuning.

The method, as described, can be used to measure the thickness of anyplanar, hollow cathode or other type of sputtering targets when thesound energy is confined into a beam directed nearly perpendicular tothe tangent to both surfaces at the beam entrance positions. This methodcan also be used for non-target applications such as aircraft engines,or for targets of varying shapes (FIGS. 3-5).

These and other features and advantages of this invention are describedin, or are apparent from, the following detailed description of variousexemplary embodiments of the systems and methods according to thisinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Various exemplary embodiments of the systems and methods of thisinvention will be described in detail with reference to the followingfigures, wherein:

FIG. 1 illustrates a schematic diagram of an exemplary method andapparatus according to the invention;

FIG. 2 illustrates a detailed schematic diagram of the method andapparatus of the invention; and

FIGS. 3, 3 a, 4, and 5 illustrate other schematic diagrams of examplesof apparatii able to be measured according to the systems and methods ofthe invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Turning to FIGS. 1 and 2, there may be seen schematic diagramsillustrating the method of instant invention. In accordance with thismethod, a test target specimen 20 of a bonded target assembly having aplanar front surface 24 and non-flat but smooth bonded surface 26 ismounted into chuck 28 of a machining tool. The machining tool rotatesthe specimen 20 axially symmetrically. Ultrasonic transducer 36 with aflat surface is mounted inside a bubbler 32. The transducer 36 ispositioned axially symmetrical with a nozzle 34 at some distance fromnozzle opening. The bubbler 32 is placed near the front surface 24 ofspecimen 20 preferably adjacent the sputter track 15 which is well knownin the art as a particular section or region of the target that is mostheavily consumed during the sputtering process. The transducer 36irradiates the front surface 24 of the test specimen 20 with a single,short-duration, megahertz frequency range ultrasonic pulse 38propagating through columns 76 and 60 of de-ionized (DI) water whichconnect the face of transducer 36 with the front surface 24 (FIGS. 1,2). Part of ultrasonic pulse 38 in the form of an echo 44, is reflectedback to the transducer 36 from the front surface 24. The other part 40of the pulse 38 propagates through the body of the specimen 20 at nearlynormal incidence to the tangent of smooth bonded surface 26, and isreflected back to the transducer in the form of echo 50. The transducer36 thus receives echoes 44 and 50, converts them into electric signals52 and 56, and sends them along the cable 58 to the gauge 59 forprocessing. By measuring the difference in time 61 it takes signal 52from the first echo 44 to occur versus the signal 56 from the secondecho 50 to occur, one can approximate the thickness of the specimen 20.Though the target specimen 20 of FIG. 1 is comprised of a generallyplanar front surface 24, the thickness of specimens having other shapes,such as U-shaped hollow cathode sputter targets having convex or concavefront surfaces bonded to a backing surface, could also be measured usingthe systems and methods described herein by reflecting echoes from thefront surface and front surface/bonded surface interface to thetransducer as described above.

As illustrated in FIGS. 1 and 2, column 60 of flowing DI water is usedto provide a path for ultrasonic pulse 38, and resulting echoes 44 and50. The nozzle 34 with the opening 62 creates the column 60 of DI waterwith a diameter in the range of 0.187 in (4.75 mm). The opening 62 isplaced most preferably at the distance of 0.020 in-0.040 in (0.5 mm-1.00mm) from the surface 24 of the specimen 20. The DI water to the bubbler32 is supplied through the inlet 70 with diameter of 0.157 in (4 mm).The inlet 70 is positioned behind the face of transducer 36. The lengthof water column 76 inside the bubbler between transducer 36 and theopening 62 is chosen most preferably in the range of 1.00-in. (25.4 mm).This length prevents detection of second echo 46 from the front surface24 before arrival of echoes 44 and 50. The nozzle 34 has a removable lid(not shown) which is used to protect transducer 36 and bubbler 32interior from contamination or damage by flying chips and cutting fluid,in the case when the bubbler is not in use. The rear part 84 of thebubbler 32 is permanently sealed to protect transducer electricconnector 86.

The bubbler 32 is mounted on a turret 90 in the position 92 originallydesignated for the cutting tool. To provide the water to the bubbler 32a plastic clear hose 94 is connected to inlet 70. The hose 94 isconnected in series with two valves 96, 98. Valve 98 is used for on/offoperation while the valve 96 is used for fine-tuning of water flow. Thetuning of water flow is necessary to form a preferably continuouslaminar flow which is free from isolated or chained air bubbles allalong the water supply system including hose 94, and columns 76 and 60.

The presently preferred transducer 34 is sold by Krautkramer-AGFA Counder the designation 113-126-340. This is an IPS style immersionnon-focused transducer with 0.250 in (6.35-mm) element size, and 10 MHzpeak frequency. The transducer 36 is electrically connected to presentlypreferred thickness gauge 59, sold by Panametrics under the designation25DL. The gauge 59, which has continuous thickness data output, isconnected through RS-232 port with a remote PC controller 110 byelectrical line 112. Controller 110 is programmed in a conventionalmanner to initiate the data acquisition event, to collect a series ofconsecutive thickness data points for several consecutive revolutions ofthe specimen 20, and finally to stop the data acquisition. Furthermore,controller 110 is programmed to process the data statistically to find aminimum thickness value. It is also programmed to display the minimumvalue and to send it to the database if required.

The method works as follows. At first, the bubbler 32 with removed lidis moved towards a measurement position. The repositioning can be donemanually or automatically. If CNC is used then the bubbler 32 mounted onturret 90 is programmatically moved to a pre-programmed X-Y position.This X-Y position is chosen to match a target position with the radiusof the deepest sputter erosion (sputter track area) 115. The X-Ypositioning is followed by positioning along the Z axis that brings thebubbler nozzle opening 62 closer to the target surface 24 with a gappreferably in the range of 0.020-0.040 in (0.5 mm-1.00 mm). During thenext step the target 20 is brought into rotation with a rotational speedpreferably in the range of 1-100 RPM. Next, the water supply valve 98 isopened. The flow of the water is adjusted by the valve 96 to eliminatethe turbulence and air bubble formation. During the next step, the PCcontroller 110 sends the command to the gauge 59 to start the dataacquisition. The gauge acquisition rate (or frequency) is set preferablyto the range of 1-50 acquisitions per second. It should be noted thatthe gauge should be preprogrammed and calibrated before the test usingknown material sound velocity and thickness ranges. The program can bestored internally in the gauge 59 memory or remotely in the controller110 memory and should be recalled for the test. The target thickness isdetermined during every data acquisition event for every position on thetarget surface. The thickness value is sent automatically to remotecontroller 110 for storage and processing. When the pre-programmednumber of thickness data points are collected preferably in the range of100-10,000 thickness data points collected, the controller 110 sendscommands to the gauge 59 to stop the data acquisition. During the nextstep, controller 110 evaluates the collected data for the minimumthickness value or other value of interest. It should be emphasized thatthe data acquisition frequency should be chosen substantially differentfrom the target RPM. In this case, a plurality of positions on thetarget surface from where the thickness data are collected would besubstantially different for every consecutive revolution of the target.Merging of all these positions for all consecutive revolutions creates afile representing a surface ring formed by a plurality of repeatedlysuperimposed positions constituting a continuous area projected on asputter track region.

Of course, (while the exemplary embodiments of the invention describedabove comprise measuring the thickness of relatively flat specimens 20bonded to smooth bonding surfaces 26, the measurement system and methodsdescribed could as well be used to determine the thickness of non-flatspecimens such as U-shaped hollow cathode sputter targets, or othershaped specimens as shown in FIGS. 3-5, for example, as is evident tothe artisan.

Turning now to FIG. 3, there is shown a hollow cathode target such asthe type set forth in U.S. Pat. No. 6,419,806 (Holcomb et al.). Theentire disclosure of this patent is incorporated by reference herein.Target assembly 300 comprises a substantially “U” cross sectionedcombination of target 302 and surrounding backing plate 304. As known inthe art, sputtering will occur from the inside surface of the target302, shown here facing the center C of the “U” or pot like target. Aflange 306 is located circumferentially around the perimeter of thebacking plate to facilitate insertion into the cathode of the sputteringequipment.

Bubbler 32 and associated column 60 of water are shown schematically.Preferably, the bubbler location is stationary with the target assemblyrotating therearound at varying rotation speed. Conversely, the bubbleritself could be moved relative to a stationary target via a robot orother programmable motion imparting mechanism.

FIG. 3 a shows an embodiment similar to 3 except the bubbler 32 ispositioned along the outside of the assembly adjacent the backing platecomponent. Here, backing plate thickness can be measured and subtractedfrom overall assembly thickness to result in determination of targetthickness. In this embodiment shown, the assembly 300 could be driven atvarying rotational speed by a mandrel or the like.

FIG. 4 shows a concave target assembly 400 comprising target 402 bondedto backing plate 404. Bubbler 32 of the type shown in FIGS. 1 and 2 ismoved along the concave target surface spaced from the target surface bya fixed dimension.

FIG. 5 shows bubbler 32 measuring the thickness of an elliptical shapedhousing 500 such as could serve as an engine block or the like. Bubbler32 is moved across outside surface 504 of the housing and measuresthickness between the outside surface 504 and inside surface 506.

In all of the embodiments shown, either the target itself and/or thebubbler may be moved relative to the other. In most cases, the target isrotated relative to the bubbler such as preferred for the embodiments of1-5. The speed of the target and the data acquisition frequency areasynchronous. In other words, at the end of the data acquisitionprocess, measurement points along the target surface are notcharacterized by fixed intervals or distances therebetween. Accordingly,a multiplicity of measurements are taken at pseudo-random datacollection locations along the target surface. The method of theinvention allows thickness measurement with very brief intervals betweenadjacent measurement locations, which locations may not be taken duringone single revolution. For example, thickness data for three adjacentmeasurement locations—one chosen, one to the left, and one to the rightof the chosen one can be collected during three different revolutions.

As a practical matter, the rotational speed of the target is changedduring the measurement time period since most commercially availablebubblers presently have fixed data acquisition frequencies. The skilledartisan will appreciate however that frequency of data acquisition couldalso theoretically be varied relative to target rotational speed inorder to provide for asynchronous relationship between target speed anddata acquisition frequency as used herein.

While the method herein described, and the form of apparatus forcarrying this method into effect, constitutes a preferred embodiment ofthis invention, it is to be understood that the invention is not limitedto this precise method and form of apparatus, and that changes may bemade in either without departing from the scope of the invention, whichis defined in the appended claims. For example, a bubbler is shown asthe preferred medium through which the sound beam is propagated toimpinge upon the target. Other media may be chosen by those skilled inthe art.

1-35. (canceled)
 36. A method of non-contact ultrasonic thicknessmeasurement of sputtering targets having a front surface and frontsurface/bonded surface interface comprising the steps of: a) securingthe target to a rotating holder and rotating said target; b)sequentially irradiating the front surface with pulses of sonic energyat a plurality of positions at substantially normal incidence to atangent line at a plurality of positions on the front surface, at leastone of the pulses propagating through the front surface to the frontsurface/bonded surface interface; c) detecting consecutive echoesinduced by said pulses of sonic energy, the echoes being reflected fromthe front surface and from the front surface/bonded surface interface,respectively; d) converting the front surface echoes and the frontsurface/bonded surface interface echoes into corresponding electricsignals; e) determining transit time of a sound path between the frontsurface echoes and the front surface/bonded surface echoes based on thecorresponding electric signals; f) determining thickness data of thetarget by multiplying known sound velocity of the target material by onehalf of measured transit time, corrected for a zero offset factor; g)electronically sending the determined thickness data to a remotecontroller; h) collecting thickness data for a plurality of positions onthe target; i) analyzing the collected data and extracting the value forthe minimum thickness of the target.
 37. The method as recited in claim1 wherein said sputter target remains attached to a rotating machiningtool chuck or other specimen holder and is rotated axially symmetricallyduring the entire thickness measurement process.
 38. The method asrecited in claim 1 wherein step (b) includes providing a bubblerassembly adjacent said front surface, said bubbler assembly including asonic energy irradiation means for irradiating said front surface withsonic energy at a data acquisition frequency.
 39. The method as recitedin claim 1 wherein step (b) includes coupling the rotating target withan ultrasonic transducer by a column of a non-turbulent bubble-freestream of water to the front surface of the target.
 40. The method asrecited in claim 1 wherein step (b) includes irradiating the target withsonic energy passing through a water column generated by an immersionbubbler.
 41. The method as recited in claim 1 wherein step (b) includesimpinging the target with sonic energy in a form of short duration, MHzfrequency ultrasound pulse.
 42. The method as recited in claim 1 whereinthe echoes are detected with a data acquisition frequency substantiallynon-synchronized with the RPM (revolution per minute) of the rotatingtarget.
 43. The method as recited in claim 1, wherein said echoes aredetected from a plurality of positions for more than one revolution oftarget under rotation.
 44. The method as recited in claim 1, wherein allthe data points of thickness data are merged into one sequential file.45. The method as recited in claim 1, wherein irradiating saidsputtering target with pulsed sonic energy occurs additionally along acircumferential path of constant radius constituted by a plurality ofall possible positions on the target front surface encompassing the ringwith the area projected into the region of deepest sputter erosion. 46.The method as recited in claim 1, wherein the sputter target is a hollowcathode sputter target having one of a convex and a concave frontsurface, and wherein the sputter target is bonded to a backing plate toform a target assembly.
 47. Apparatus for measuring thickness of asputtering target bonded into an assembly comprising: a transducer forsequentially irradiating a sputtering target with sonic energy and fordetecting echoes induced by said sonic energy; an enclosed immersionbubbler, encompassing the transducer, mounted axially, symmetrical witha nozzle of said bubbler at normal incidence to a surface of the target;a thickness gauge electrically connected to said transducer andprogrammed to measure a sound transit time between two consecutiveechoes, and programmed to calculate the specimen thickness, based onknown sound velocity, transit time, and zero offset factor; a controllerelectrically connected to the thickness gauge and programmed: a) to senda trigger command to start the data acquisition; b) to receive andcalculate values of the thickness as soon as thickness data collectionis completed; c) to merge each calculated thickness value into onesequential file; d) to send a trigger command to stop the dataacquisition; e) to analyze the data, determine and display the value forthe minimum target thickness.
 48. The apparatus as recited in claim 12wherein said bubbler is physically spaced from the target front surfaceso as to not contact physically the front surface of the target, andsaid bubbler includes a nozzle having an opening providing fullytransparent transmission of sound energy to and from the transducer. 49.The apparatus as recited in claim 12 wherein said transducer ispositioned inside the bubbler a distance from the nozzle opening toprevent receipt of an interfering echo prior to receipt of consecutiveechoes from the front surface of the target and from a frontsurface/bonded surface interface of the target, respectively.
 50. Amethod for making non-contact ultrasonic thickness measurement of acomponent of a sputter target assembly comprising the steps of: a)positioning the assembly on a mounting member; b) providing a source ofsonic energy irradiation adjacent a surface of said component forirradiating said surface with sonic energy at a data acquisitionfrequency; c) providing movement of said sputter target assemblyrelative to said source of sonic energy irradiation and asynchronouslyrelating said movement relative to said data acquisition frequency; d)sequentially irradiating the surface of said component with pulses ofsonic energy from said source of sonic energy irradiation; e) detectingconsecutive echoes induced by said pulses of sonic energy; and f)determining thickness of said component by determining time intervalsbetween said consecutive echoes.
 51. A method as recited in claim 15wherein said component comprises a target, said target assembly furthercomprising a backing plate bonded to said target, said target having afront surface and a back surface bonded to said backing plate, said step(d) comprising irradiating said front surface of said target and whereinsaid consecutive echoes include, sequentially, echoes from said frontsurface and echoes from said back surface.
 52. A method as recited inclaim 15 wherein said step (c) comprises rotating said target assemblyaround a fixed position bubbler assembly.
 53. A method as recited inclaim 15 wherein said step (c) comprises varying the rotational speed ofsaid target assembly during said measurement.
 54. A method as recited inclaim 15 wherein said data acquisition frequency comprises fixed equalintervals.
 55. A method as recited in claim 15 wherein said targetassembly has a substantially “U” shaped cross section.